Waste heat boilers are commonly used with many types of industrial heat sources to extract heat from waste gases of an industrial process. It may be necessary to extract heat from the waste gas to cause a component thereof to condense, or it may be advantageous to extract heat from the waste gas and use that heat in another process or even to provide heat for the industrial facility.
Generally speaking, a waste heat boiler includes a plurality of metal boiler tubes supported by opposed metal tube sheets. The tube sheets define a vessel for holding water or some other form of heat transfer medium. Hot waste gas passes through the boiler tubes arranged in the inlet tube sheet and heat is extracted therefrom via heat transfer from the hot gas to the water contained within the confines of the tube sheets.
It is well known that it is necessary to shield the inlet ends of the boiler tubes from the heat and corrosive nature of the waste gas. It is therefore common to employ ceramic ferrules, such as those disclosed in U.S. Pat. No. 4,176,612, in the inlet ends of the boiler tubes. As shown in FIG. 1, a portion of each ceramic ferrule 10 extends out of the inlet face of each boiler tube 22. A castable refractory wall 30 is formed around the ferrules to hold the ferrules in place and to provide a heat shield for the inlet tube sheet 24. Thus, the ceramic ferrules and the castable refractory wall respectively shield the inlet ends of the boiler tubes 22 and the inlet tube sheet 24 from the waste gas.
There are several problems with this type of conventional ferrule/refractory wall system. First, it is extremely labor intensive to form the refractory wall 30 around the ferrules 10 extending out of the inlet tube sheet 24. Second, since the refractory wall 30 is typically several inches thick, its weight induces significant stresses on the ferrules 10, thus requiring that the thickness of the ferrule walls be relatively large. Larger ferrule wall thickness equates to lower throughput (i.e., lower open area and higher pressure drop) of waste gas through the waste heat boiler. Third, the refractory wall 30 and ferrules 10 become a monolithic body after the castable refractory sets. Such a large refractory body does not handle thermal cycling well and will crack after repeated heating/cooling cycles. The cracked refractory wall/ferrule body requires regular repair and replacement.